As described in U.S. Pat. No. 5,150,247 (hereinafter referred to as the '247 patent), issued Sep. 22, 1992, to R. Sharpe et al, entitled "Fiber Optic Telecommunication System Employing Continuous Downlink, Burst Uplink Transmission Format with Preset Uplink Guard Band," assigned to the assignee of the present application and the disclosure of which is incorporated herein, fiber optic (or fiber-in-the-loop (FITL)) communication systems have been proposed as a broadband replacement for a variety of communication networks, such as traditional `copper wire` telephony networks and `add-on` cable television distribution networks (which employ separate, dedicated coax conductor cables).
Unfortunately, the cost of implementing the systems that have been proposed to date for broadband networks is relatively high, due to the fact that such schemes often involve the use of a large number of electro-optic and interconnect components, such as source and detector elements. When added to the cost of optical connectors and splices for the fiber optic links, and the quantity of optical fiber itself in the distribution cable between opposite ends of the link, such a large number of components severely impacts overall system expense. As a result, on a per-subscriber basis, the aggregate cost of the components is a large fraction of the price tag of an overall system, which, in addition to employing two transmitter/receiver pairs per subscriber, customarily uses a separate, dedicated pair of optical fibers within the distribution cable for each subscriber.
To reduce such a costly outlay for fiber optic components needed for the system, one approach has been to design signal processing and communication architectures that can handle a very large number of subscriber lines per unit and employ a highly compressed data format. Unfortunately, such an approach is effectively self-defeating, since, in order to spread out the cost of the fiber optic components over a large number of subscribers, the system architecture becomes highly complex and therefore aggravates rather than alleviating the cost problem.
To solve this shortcoming, the fiber optic telecommunication scheme described in the '247 patent employs a time division multiplex communication scheme that integrates plural television and telephony signals in a continuous time division format in the downstream direction, and integrates channel selection requests and telephony signals from the customer premises equipment in a slotted burst format in the upstream direction.
More particularly, referring to FIG. 1, which diagrammatically illustrates the '247 patent approach, a host digital terminal (HDT) at a master site 10, to which both telephony and multiple channels of video signals are supplied, is linked to a plurality of remote, geographically separated customer premises 40 by means of a fiber optic cable pair 20. Downstream-directed telecommunication messages (containing both telephonic signals, such as those supplied by way of a central office switch 14 via a link 12, and scrambled television channel-representative digital signals 18, supplied via a link 16) are transmitted in a continuous-mode time division multiplex format over a first optical fiber 21 from the host digital terminal at the master site 10 to a downstream-located pedestal 23.
Pedestal 23 includes a multiple fan-out, optical splitter device 26 located at a first common `split` point on the downlink fiber 21, and multiple feed-in optical combiner device 27 located at a common `combine` point on a second, uplink fiber 22. Through these fiber coupling connections within the pedestal, the downlink and uplink fiber pair 21, 22 are coupled by respective downlink and uplink fiber links 24, 25 to a plurality of subscriber interface optical network units 30, that service one or more associated customer premises 40 through associated broadband service drops 35.
The respective television channels within downlink-directed, digitally formatted telecommunication signal frames transmitted over the downlink fiber 21 from the master site 10 correspond to those selected in response to subscriber channel requests contained within uplink burst messages that have been transmitted over the uplink fiber 22 from the customer premises equipments. Each downlink message identifies the optical network unit 30 for whom the message is intended and specifies which digital subscriber line packets are directed to it. The recipient optical network unit 30 demultiplexes the contents of the message and routes the respective television channel signals and telephony signals to customer premises equipment (CPE), such as a TV set top decoder 41 associated with television set 43, telephone handset, etc., at the destination site 40.
Upstream-directed, channel request burst transmissions from the customer premises 40 to the master site 10 occur in time division multiple access format. Within preassigned time slots of successive uplink burst cycles, the subscriber sites are also given the opportunity to transmit a digital subscriber line data packet containing samples of telephony signals. Each uplink burst cycle may contain additional time slots that are accessible by the subscriber interface sites on a contention basis, for network control and television-related signalling.
To prevent collisions between successive uplink slotted signal bursts, a prescribed guard band is used to separate successive uplink time slots from one another. The nominal duration of the guard band accommodates the longest optical fiber transmission distance difference from the common `combine` point on the uplink optical fiber and the various subscriber interface sites.
Now although the communication scheme described in the '247 patent offers a significant improvement over previously proposed broadband systems, the continuous time division multiplex format it uses for downstream-directed messages, and the slotted burst upstream format employed for upstream-directed messages may not necessarily be preferred by all service providers. Indeed, since the inception of the '247 scheme, the broadband communications industry has devised internationally accepted standards that are well suited for the transport of a wide variety of data types, in particular synchronous optical network transport stream (STS)-based transmissions, in which asynchronous transfer mode (ATM) data cells are conveyed, while still allowing for the use of a slotted bus return channel from the customer premises equipment.
With this end in mind, co-pending patent application Ser. No. 08/383,984, (hereinafter referred to as the '984 application), filed Feb. 6, 1995, by R. Sharpe et al, entitled "Point-to-Multipoint Broadband Services Drop with Multiple Time Slot Return Channel for Customer Premises Equipment Served by Fiber Optic Telecommunication System Employing STS-based Transmission Format Containing Asynchronous Transfer Mode Cells," assigned to the assignee of the present application and the disclosure of which is also herein incorporated, describes a modification of the system described in the '247 to accommodate the use of such communication protocols. Specifically, the '984 application describes the use of an ATM standard, in which STS-based signals are broadcast to a plurality of customer premises equipments, with a selected portion of an STS-based signal employed by an optical network unit being used to generate a timing reference, through which associated customer premises equipments along a drop are provided with uplink burst timeslot assignment. The transport overhead portion of an STS-1 frame is followed by a synchronous payload envelope (SPE), with ATM transport standard cells encapsulated in the SPE.
The optical network unit provides for an ATM cell-embedded STS signalling format. The downlink transceiver end of the optical network unit, which is interfaced with an optical receiver coupled to the downlink fiber link from the pedestal, is applied to a demultiplexer that demultiplexes a single telephony/signalling channel into a telephony/signalling demultiplexer portion of a mux/demux unit and a plurality (e.g. twenty-four) video channels to a standard ATM interface.
The particular video channels being transported over the fiber correspond to channel requests from customer premises equipment served by the system. To direct selected (customer-requested) ones of a plurality of N input channels provided at the upstream end of the link onto selected ones of a plurality of M output channel slots (associated with the number of customers served by the system) transported in the downstream direction to the customer sites served by the system, an N.times.M multiplexer may be installed at the upstream site. As described in the '247 patent, such an N.times.M multiplexer is coupled to receive a large number (e.g. sixty-four) of parallel supplied channel inputs and, in response to control signals that are based upon downstream customer requests, directs one or more of those parallel inputs to one or more of the output channel slots for delivery to a customer site.
Now, for a large market share television program, such as the Super Bowl, where a significant number of customer sites can be expected to request the same channel/program, the input of the N.times.M multiplexer receiving that (Super Bowl) channel will be repetitiously steered or routed to a large number of output channel time slots, thereby using a large portion of bandwidth for transport of the same program. In addition, regardless of which channels are to be transported, the use of such an N.times.M multiplexer at the upstream end of the link, and an associated demultiplexer at the downstream end of the link require an addressing scheme indicating which customer equipment has sourced a channel request (and to whom the requested channel is to be transported), as well as steering the correct channel to the customer.